Torque is rotational force, and represents a rate of change of angular momentum of an object. In the International System (SI), torque is measured in newton-meters (N·m), and angular momentum is measured in newton-meters-seconds (N·m·s). Angular momentum is proportional to the rotational inertia of the object times its angular speed. The angular momentum of a system remains constant, unless acted on by an external torque. The change to the angular momentum due to application of torque is called angular impulse (also N·m·s). For example, an object that is not rotating can be accelerated to a spin having an angular momentum of “x” N·m·s by application of a torque of “x” N·m for one second, equivalent to applying an angular impulse of “x” N·m·s.
“High-torque” application tools abruptly apply a large peak torque to an output shaft, resulting in rotational force jumping near-instantaneously from zero to a large value. Plotted against time, each application of torque would graphically appear to be a “spike,” jumping from no torque to a large peak and then returning to no torque. Since the tool possesses rotational inertia, this quick spike of force reduces the exertion required by the user holding the tool, relative to the resistance the user needs to provide if the force was sustained continuously over a longer period of time.
Two common types of high-torque application tools are impact drivers and pulse torque tools. An impact driver (commonly referred to as an impact gun) is designed to deliver high torque output by storing energy in a rotating mass (e.g., the hammer), which is impacted to suddenly connect the rotating mass to an output shaft (e.g., the anvil). After delivering the impacting force, the hammer again spins freely from the anvil. Pulse torque tools use oil or other hydraulic fluid with a clutch to transfer kinetic energy from the hammer into the anvil to produce torque. By repeatedly applying the impacting torque to the hammer, impact drivers and pulse torque tools produce a series of impacting force-pulses over time, with torque returning to zero between each spike of force.